> identify any of their de-identified data that came from their usage of ChatGPT

"de-identified" seems more of a euphemism than normal in this context, given the very unique work they were doing.

I wouldn't expect poking at millennium problems to be that rare in ChatGPT. They were uniquely successful - but it's probably not easy to check de-identified data for the presence of any of their work on the problem because it would blend into a haystack of less successful work on the problem.